Preheat oven to 350°F. Grease an 8 by 8-inch baking dish or pan and set aside.
Melt the butter in a large, microwave-safe bowl. Use a whisk to stir in the brown sugar until well-combined. Beat in the egg and vanilla until completely incorporated. Blend in the flour and salt until fully blended. Fold in half of the chocolate chips or other add-ins.
Scrape the batter into the prepared dish or pan, smooth the top, and sprinkle with the remaining chocolate chips/add-ins. Bake for 20 to 25 minutes or until the center is set. Set the pan on a wire rack to cool completely before slicing.
